1. Field of the Invention
The invention relates to a monomode optical fibre provided for transporting light having a wavelength xcex with 480xe2x89xa6xcexxe2x89xa6550 nm. The fibre comprises a core made of a first transparent dielectric material, having a first refractive index nc and a substantially circular cross-section with a radius dimension a and a first cladding coaxially applied on said core and made of a second transparent dielectric material having a second refractive index nm1, wherein nm1 less than nc.
2. Prior Art
Such a monomode optical fibre is known from U.S. Pat. No. 3,997,241. The known optical fibre has a core surrounded by the first and a second cladding. The first refractive index being higher than a second refractive index of the second cladding. The purpose of using two different materials with respective refractive indices is to reduce the transmission loss occurring when the fibre is bent. The use of a monomode optical fibre is imposed by the constraint that the spatial coherence of the transmitted laser light should be maintained.
A drawback of the known optical fibres is that there is a severe limitation inhibiting a transport of multi-watt laser light. This limitation is caused by a non-linear optical effect, called Brillouin scattering threshold, imposing a threshold PB beyond which the light is no longer transmitted by the optical fibre.
It is an object of the present invention to realise a monomode optical fibre enabling an efficient light transport even with light intensities hi her than 4W.
For this purpose, a monomode optical fibre according to the present invention is characterised in that the first transparent dielectric material consists in substantially pure silica, the second transparent dielectric material consists in doped silica, the thickness of the said first cladding is comprised in a range of 5 to 15 xcexcm, the values of nc and nm1 are chosen in such a manner that the numerical aperture (NA={square root over (nc2xe2x88x92n2m1)}) is less than 0.1 and the radius dimension a is above 2 xcexcm.
Silica is particularly suitable for optical fibre and can easily be doped. The use of doped silica for the second material enables to select an adequate doping substance in order to obtain the required numerical aperture. Moreover, the combination of doped silica with pure silica enables to easily combine the different subsequent materials.
The Brillouin scattering threshold is mainly determined by the cross-section of the core and the effective length of the fibre. Since a monomode fibre is used, the following equation has to be satisfied:
2xcfx80a NA/xcex less than 2.401 
where xcex is the wavelength of the transported light. So by choosing the refractive indices of the two transparent materials in such a manner that NA less than 0.1, it is possible to increase the radius a of the core without getting into conflict with the above mentioned equation. Since the Brillouin scattering threshold is mainly determined by the cross-section of the core, an increase of the cross-section enables to raise that threshold value and consequently the power of the transmitted light. The composition of the fibber core, consisting of substantially pure silica, helps permitting transmission of high power through the fibber according to the invention without lowering the core transparency and consequently the transmissivity of the fibber due to absorption of energy in the said core, as with a fibber core made for example of doped silica. The considered purity is maintained namely due to the low thickness of the first cladding of the fibber, made of doped silica. Indeed, such a low thickness avoids migration of doping agents from the cladding into the core of the fibber, especially during the manufacturing process of the latter.
A first preferred embodiment of a monomode optical fibre according to the present invention is characterised in that the doping agent of the said second transparent dielectric material is chosen amongst fluorine and boron. These doping agents conveniently enable the adjustment of nm1, without migrating into the fibber core so that to damage its transmissivity properties when transmitting energy, as it is the case with other doping agents, such as OH ions.
Preferably, the doping agent in the second transparent dielectric material is present in a molar concentration range of 1 to 3 percents. Such concentrations enable to obtain a low NA value without leading to risk of diffusion of doping agent into the fibber core.
In a second preferred embodiment of a monomode optical fiber according to the present invention, the numerical aperture NA of the fibber is adjusted in order to be above 0.03, preferably above 0.04, most preferably above 0.05. Such values are convenient to obtain a monomode optical fibber having a high Brillouin scattering threshold and whose core can present normal micro bending without enduring transmitted power leakage as with fibbers having very low NA values, i.e. below 0.03. A third preferred embodiment of a monomode optical fibre according to the present invention is characterised in that said fibre further comprises a second cladding coaxially applied on said first cladding and made of a third transparent dielectric material having a third refractive index nm2, wherein nm2 greater than nm1. The application of a second cladding enables to limit the dimension of the first cladding without affecting the characteristics of a cladding.
Preferably said first refractive index nc and said third refractive index nm2 have a same value. In such a manner, the second cladding has the same refractive index as the one of the core and enables an easy manufacturing, preferably with the same material, that is substantially pure silica.
A fourth preferred embodiment of a monomode optical fibre according to the present invention is characterised in that said fibre in enveloped with a resilient material in such a manner as to limit the bending radius of the fibre to minimum 5 cm. By limiting the bending radius of the fibre, losses due to excessive bending are limited.
A fifth preferred embodiment of a monomode optical fibre according to the present invention is characterised in that an end-wall of the fibre has an inclined end-face having an inclination angle xcex8  θ   greater than             1      2        ⁡          [                        π          2                -                  a          ⁢                      xe2x80x83                    ⁢                      sin            ⁡                          (                                                n                  c                                                  n                  m                                            )                                      +                  a          ⁢                      xe2x80x83                    ⁢                      sin            ⁡                          (                              NA                                  n                  c                                            )                                          ]      
By imposing such an inclination angle, it is avoided that light reflecting against the end-wall would be reflected back into the fibre and would thus perturb the light transmitted through the fibre.
Preferably, an extremity of the fibre is provided with an end-piece having a cavity for receiving said extremity, a bottom of said cavity being provided with a hole through which said core extends. This enables an easy coupling of the fibre to the light source.